The principal aim of the Section on Cellular Biophotonics is to use imaging techniques, such as two-photon microscopy, spectral imaging, fluorescence lifetime microscopy, and fluorescence anisotropy analysis to study how protein complexes regulate synaptic function in living cells. Recently, we have concentrated our efforts on utilizing Forsters Resonance Energy Transfer (FRET) to monitor protein-protein interactions. This method has great potential for studying protein interactions because it is sensitive to changes in the distance separating two fluorophores on the 1-10 nm scale. FRET imaging in conjunction with the development of spectral variants of Green Fluorescent Protein (GFP) provides the opportunity to genetically tag synaptic proteins of interest and monitor their interactions with other labeled proteins in real time. Our Section is comprised of Drs. Steven Vogel (Chief), Srinagesh Koushik (Research Fellow), Christopher Thaler (Postdoctoral IRTA), and Jose Fernando Covian-Nares (Visiting Fellow). [unreadable] Our Sections initial efforts concentrated on 1. Building and testing a laser scanning microscope specifically designed for studying protein-protein interactions in living cells, 2. Develop new methods for measuring FRET, and 3. Overcoming some of the practical limitations of FRET imaging. The microscope we have constructed is a fully functional laser scanning two-photon microscope, with the additional capabilities of measuring florescent emission spectra (spectral imaging), fluorescent lifetime decays (FLIM), and fluorescent anisotropy lifetime decays (rFLIM). These added capabilities make it specifically useful for monitoring FRET between either dissimilar (Hetero-FRET) or similar (Homo-FRET) fluorophores. Since our last report we have published one new manuscript using this imaging system where we develop the first FRET standards for validating FRET experiments (Cerulean, Venus and VenusY67C FRET reference standards. S. V. Koushik, H. Chen, C. Thaler, H.L. Puhl III, and S.S. Vogel. Biophysical Journal 91:L99-101 (2006)). We have provided these FRET standards to over 40 research groups including laboratories in Germany, Spain, Canada, Netherlands, Austria, Singapore, Switzerland, and Denmark. Additionally we have submitted for publication a book chapter (Spectral Imaging and its use in the measurement of Frster resonance energy transfer in living cells. S.S. Vogel, P.S. Blank, S.V. Koushik, and C. Thaler) for a text book on FRET imaging, and two additional manuscripts, (Two independent forms of compensatory endocytosis maintain embryonic cell surface homeostasis during early development. J. F. Covian-Nares, R.M. Smith, and S.S. Vogel) and one manuscript now in revision (Energy migration alters the fluorescence lifetime of Cerulean: Implications for FLIM-FRET measurements. S.V. Koushik, and S.S. Vogel. [unreadable] [unreadable] Currently we have 5 working projects in the lab. The first project is involved in concluding our feasibility and methodological studies on FRET imaging. The last four projects initiate the next phase in our Sections activities where our microscopes unique capabilities are utilized to address biological questions:[unreadable] [unreadable] 1. We have generated Homo-FRET reference standards that will be used in interpreting anisotropy decay experiments. This relatively new method has the potential for monitoring how proteins form multimeric structures, and their stoichiometry in living cells.[unreadable] [unreadable] 2. The second project has been following up on an observation we have made where we measure more and faster FRET between GFP type fluorophores than are predicted by FRET theory. After consulting with FRET experts from around the world, our current working hypothesis is that because GFP fluorophores are protected from the external environment by its beta-barrel protein structure, vibrational dequenching of excited states are slowed and thus allow for coherent energy transfer between FPs. If our hypothesis is correct, this would be one of the first examples of a biology adaptation to exploit quantum mechanical behavior at room temperature. This phenomenon might also be useful for generating components required for building quantum computers.[unreadable] [unreadable] 3. Our third project uses anisotropy lifetime decay analysis to monitor changes in the multimeric structure of Cam kinase-II. This abundant synaptic enzyme has been shown to play a pivotal role in learning and memory. It is believed that long lived structural changes in this protein complex might be the embodiment of some forms of memory. Our results indicate that structural changes associated with Cam kinase-II activation can be detected with anisotropy imaging.[unreadable] [unreadable] 4. In our fourth project we are using two-photon microscopy to investigating the roles of Src kinase (an oncogene) and Dynamin (a protein that assembles into a coiled structure and is directly involved in membrane scission ) in regulating endocytosis and cell division. We have developed a new imaging based assay for testing the effects of either over expression of exogenous proteins, or down regulation of endogenous proteins, on compensatory endocytosis and cell division in developing sea urchin embryos. Using this assay we have found a surprising connection between endocytosis and cell division. Over expression of Src kinase, inhibits compensatory endocytosis and blocks cell division. Inhibitors of tyrosine phosphatase had similar effects, suggesting that the balance of tyrosine kinase and phosphatase activity plays a key role in how cells regulate endocytosis and cell division. Injection of an anti-sense morpholino against Dynamin also blocked endocytosis and cell division, and treatment with a peptide that blocks Dynamin s interactions with other proteins through its SH3 (Src homology 3) domain also blocked endocytosis and cell division. Injection of an anti-sense morpholino against Src inhibited cell division, but did not inhibit endocytosis. These experiments suggest that dynamin-dependent endocytosis is required for cell division.[unreadable] [unreadable] 5. Finally, the Jain foundation has generously established a gift fund in support of our laboratories preliminary investigations into the functions of Dysferlin, a protein that is known to be responsible for Myoshis muscular dystrophy.